This invention relates generally to current-limiting fuses. More particularly, this invention relates to connectors for current-limiting fuses suitable for use in high-voltage applications.
Over-current protection may be provided by fuses as well as by circuit breakers, switches, relays and other devices. Each type of equipment has variations in ratings, service requirements and costs. Fuses generally present the most cost-effective means for providing automatic high-voltage current protection against a single over-current failure. Most types of fuses are designed to minimize damage to conductors and insulation from excessive current.
High voltage current-limiting fuses are used in a variety of applications. The basic fuse construction consists of a fusible element, a core to support this element, filler for enhancing the interruption of fault current at high voltages, and a housing to house the above components. There are provisions to connect the fuse to an external electric circuit, typically located at each end of the fuse. The fuse housing materials may consist of glass, ceramic, porcelain, and glass-filament-wound epoxy tubing. Copper ferrules or sand cast caps are typically glued to the ends of the fuse body with an epoxy or pressed onto the fuse housing with an interference fit to form end enclosures.
Fuses protect against over-currents in electrical equipment. The current path within a typical fuse is through the end caps or ferrules to a metallic fusible element. The resistance of the fusible element develops heat that causes a portion of the metal to melt or disintegrate upon reaching the melting temperature of the metal. This property is exploited to achieve accurate thermal activation of a fuse in response to a particular level of overload current. The thermal activation exhibits an inverse-time response curve. In other words, a small overload generally takes a longer time to heat the metal and melt the fuse. As the overload current increases, the heating and melting time is reduced.
The physical length of a high-voltage fuse with a fusible element of a given length is reduced by winding the element spirally around a core. In order to impede arcing in high-voltage applications, a non-conductive filler material is typically used to fill the voids between conductive portions of the fuse to quench the arcing.
A typical high-voltage current-limiting fuse comprises a tubular insulating housing, an elongated core within the housing, and one or more fusible elements wound about the core and connected between terminals at opposite ends of the housing. A core is needed in fuses rated at 5 kilovolts (xe2x80x9ckVxe2x80x9d) and above in order to enable the fuse to accommodate the required length of fusible element within a housing of practical length. Typical housing lengths range from 8 to 38 inches for voltages up to about 46 kV. By winding the fusible elements about the core, preferably in a generally helical path, fuses having fusible elements of a length much greater than the length of the core can be produced.
In prior art high-voltage fuses; the cores are typically made of mica, or of a ceramic material that may not have gas-evolving properties. These cores typically have a transverse cross-section in the shape of a star, i.e., with a centrally located trunk and a plurality of legs projecting from the trunk, with recesses between the legs, as is illustrated, for example, in U.S. Pat. No. 4,028,655 to Koch et al. One reason for using this core configuration is so as to lengthen the creepage distances along the core surface between the turns of the fusible element(s). In the manufacture of such fuses, the fusible elements are helically wound about the star-shaped core, and the resulting assembly is inserted into the tubular housing. The housing is then filled with particulate matter, typically silica sand, which is densely packed about the core-fusible element assembly and also in the recesses between the core legs and the fusible elements. To assist in packing the sand with the desired high degree of density, the fuse is typically vibrated during and after being filled with the sand. The star shape of the core makes it difficult to achieve the desired high density of the fill since vibration for a long period of time is needed to achieve a dense pack of sand in the recesses between the core legs and the fusible elements.
The performance of such a fuse depends in part upon the sand fill being held in close proximity to the location of the fusible elements since the arc or arcs formed upon operation of the fuse need to quickly react with and to be effectively quenched by the surrounding sand in order for the fuse to effect the desired current-limiting action. In the typical prior art fuse, this close proximity between the sand and the fusible element(s) is achieved by densely packing with sand the otherwise vacant spaces about the fusible element(s), including the recesses between the core legs. In view of the difficulties involved in packing these recesses with the sand fill, it would be highly desirable if the close proximity required between the sand and the fusible elements(s) could be achieved without the need for providing such recesses in the core for receiving the sand fill.
A cylindrical sand core has been shown to facilitate dense packing of the filler material to thereby improve the consistency of manufacture and the anti-arcing properties of the resulting fuses. A fuse comprising such a sand core has been described in U.S. Pat. No. 5,670,926 to Ranjan et al, which shares common inventorship with the present invention.
One disadvantage of the prior art is that high-precision assembly techniques and/or intensive manual labor have been required to consistently locate the fusible element centrally within the filler material in order to reduce undesirable arcing. The high precision techniques and manual labor each lead to increased manufacturing costs when done properly, or unreliable arcing within the fuses in some other instances.
U.S. Pat. No. 4,506,249 to Huber shows a method for terminating the fuse element in a current-limiting fuse. This teaching is directed to supporting a mica core and a method for terminating the corresponding fuse elements. Unfortunately, a sand core is heavier than a mica core and requires different types of element terminations and connections.
In an exemplary embodiment of the invention, the above-discussed and other drawbacks and deficiencies are overcome or alleviated by a fuse having an elongated housing, an auto-centering connector centrally located about the elongated axis of the housing, and an elongated fusible element attached to the connector and secured along the elongated axis of the housing by the connector.
The auto-centering connector is located about the elongated axis of the housing by a surface near one end of the housing, and the elongated conductive fusible element is connected to the connector and contained within the housing. The auto-centering connector is connected to an end of the fusible element, and mechanically located but electrically insulated relative to the housing.
These and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.